(a) Field of the Invention
The present invention relates to a method for manufacturing a solid electrolytic capacitor and, more particularly, to a process for connecting a cathode conductor layer with a cathode lead in a solid electrolytic capacitor.
(b) Description of the Related Art
Some solid electrolytic capacitors use a valve-function metal such as tantalum. Fabrication of such a solid electrolytic capacitor (may be referred to as electrolytic capacitor or simply capacitor hereinafter) includes a step for connection between a pair of electrodes, which include an anode electrode and a cathode conductor layer, and a pair of external lead wires, which include an anode lead and a cathode lead. In a conventional fabrication process, the connection between the anode electrode and the anode lead is usually effected by electric welding. On the other hand, the connection between the cathode conductor layer and the cathode lead is effected by several methods, which involve problems to be solved. A variety of proposals and studies have been made to solve the problems.
Examples of the connection between the cathode conductor layer and the cathode lead include the use of a conductive adhesive, such as a silver paste, wherein conductive fillers such as silver particles and an adhesive such as epoxy resin are admixed. FIG. 1 shows a sectional view of an example of conventional solid electrolytic capacitors fabricated by using the conductive adhesive. The capacitor comprises a capacitor body 11, an anode electrode 12 protruding from the top surface of the capacitor body 11, and a conductor layer 13 constituting a cathode electrode formed on the side and bottom surfaces of the capacitor body 11. The cathode lead 16 is electrically connected and fixed to the cathode conductor layer 13 by a conductive adhesive 23.
The conventional capacitor of FIG. 1 requires separate steps for connection on the anode side (between the anode lead 15 and the anode electrode 12) effected by electric welding and for connection on the cathode side (between the cathode lead 16 and the cathode conductor layer 13) effected by the conductive adhesive 23, resulting in a lower throughput of the capacitors. In addition, the connection on the cathode side requires a heat treatment for a certain length of time, which further lowers the throughput of the capacitors.
Examples of the connection on the cathode side include the use of a soldering technique. A soldering technique using a pulse thermal treatment is described in Publication NO. JP-A-1990-46715, which includes in essence the steps of mounting the capacitor body on a pre-heated hot plate, and removing the capacitor body therefrom after a given length of time to thereby apply a pulse thermal energy to the capacitor body. The waveform of the pulse thermal energy has a rapid rise, a rapid fall and a high amplitude.
The soldering connection on the cathode side requires separate operations for the anode side and cathode side, similarly to the case of the conductive adhesive. Also, the soldering technique requires a relatively large length of time, for example, several seconds, for the pulse heating, which also lowers the throughput of the capacitors.
A proposal is made which uses the electric welding for the connection on the cathode side. Examples for the electric welding include a parallel gap welding in which welding current flows between a pair of parallel welding electrodes disposed in the vicinity of the connection. FIG. 2 shows a top plan view of the capacitor during the parallel gap welding, which is disclosed in UM Publication No. JP-B-1987-2762. A cathode lead 16 running parallel to the bottom surface of the capacitor body 11 in spaced relationship therewith is connected to the capacitor electrode (not shown) by a fuse 24, which is welded by the parallel gap welding technique for bridging the cathode lead 16 and the cathode electrode. The parallel gap welding using the electric welding, however, generally requires a relatively large space for the welding, which is not suited to a small-sized product.
Another electric welding known in other technical fields applies a pressing force between a pair of welding electrodes sandwiching therebetween an element to be welded. If this technique is used in fabrication of a capacitor, the pressure is applied directly to the capacitor body to thereby damage the insulator film etc. of the capacitor body thereby increasing the leakage current between the electrodes. Accordingly, the characteristics or reliability of the capacitor is lowered.